Today, radiation therapies for cancers are rapidly developing centering around various irradiation methods, and along with the development, the importance of measurement of three-dimensional absorbed dose is increasing. To evaluate radiation absorbed by the living body, it is necessary to use a dosimeter sensor having the same effective atomic number as that of biological tissues. Dose measured with a sensor having a different effective atomic number cannot be used to measure dose absorbed by biological tissues accurately.
A two-dimensional dose distribution is now obtained by Gafchromic film or imaging plate (IP) photoreceptor. However, since Gafchromic film can be used only once, in-plane sensitivity coefficient cannot be obtained, nor can disrupted images resulting from uneven coating of photoreceptors be corrected; hence, Gafchromic film has problems in quantitative capability. Further, Gafchromic film has a small dynamic range and this problem imposes many restrictions on use of the film. Meanwhile, since IPs are not biological tissue equivalent, it is virtually impossible to apply IPs to three-dimensional measurement. A method of measuring three-dimensional dose distribution using a molded product of a polymer gel in which a biological tissue-equivalent fluorescent substance is dispersed is also being studied, but the method is highly burdensome in terms of facilities and labor and is not practical.
Known phosphors that can be used for measurement of radiation dose are thermoluminescent phosphors, which emit light by heating after irradiation with radiation (for example, Non-patent Document 1) and photostimulable phosphors, which emit light by light irradiation after irradiation with radiation. Patent Document 1 discloses a sheet-like two-dimensional dosimeter obtained by mixing a lithium fluoride-based thermoluminescent phosphor and a binder and hot-pressing the mixture in a die. Lithium fluoride loses thermoluminescence when it is exposed to a high temperature; hence, a conventional problem is that when a lithium fluoride-based thermoluminescent phosphor is mixed with a binder and the mixture is heated and processed into a sheet, the thermoluminescence decreases. In this regard, in the invention disclosed in Patent Document 1, it is stated that the foregoing problem has been resolved since a tetrafluoroethylene-ethylene copolymer is used as a binder which is cured by heating at a relatively low temperature of 260° C. However, the effective atomic number of a lithium fluoride thermoluminescent phosphor is 8.2, which differs from the effective atomic number of biological tissues, 7.4, for photoelectric effect. Since the total effective atomic number of a sheet comprising a fluorine resin used as a binder is larger, the biological tissue equivalence of the sheet is not satisfactory.
On the other hand, a dosimeter using a photostimulable phosphor is superior in rapidity and hence, there is a demand for a photostimulable phosphor for obtaining a two-dimensional or three-dimensional dosimeter that is superior in handleability, biological tissue equivalence and precision. Non-patent Document 2 discloses that the phase transition temperature of lithium heptaborate and lithium tetraborate+liquid phase is 856±2° C. in a mixed component phase of a lithium oxide and a boron oxide (3:1). However, the document does not refer to a silver-containing lithium heptaborate photostimulable phosphor.